1. Field of the Invention
The present invention concerns an apparatus for spatial modulation of an x-ray beam, of the type having a number of planar attenuation elements for x-ray radiation that are disposed in a grid-like manner on a carrier and that, independently of one another, can be pivoted or tilted piezoelectrically between at least two positions. The invention furthermore concerns an x-ray image system with such a modulation apparatus as well as different methods for operation of such an apparatus.
2. Description of the Prior Art
Laminar x-ray image systems are used primarily in medical diagnostics in order to acquire radiographic images of the inside of the body of a patient. The patient is penetrated by an x-ray field extending perpendicularly in two dimensions to the propagation direction, and the spatially dependent attenuation of the x-ray radiation received behind the patient is represented or evaluated as image information. In addition to conventional radiography, laminar x-ray imaging systems are used in fluoroscopy as well as, more recently, in so-called multi-slice systems in computed tomography.
The radiation dose to which the patient as well as the medical personnel is exposed during the examination plays a significant role in applications in medical x-ray diagnostics. A reduction of the applied x-ray dose can be achieved by use of a semi-transparent pre-filter that has a central opening for the unattenuated passage of x-ray radiation therethrough. By suitable placement of such a filter as is known, for example, from U.S. Pat. No. 5,278,887, only the region of the patient within the two-dimensional radiation field is charged with the necessary dose that is of interest for the user of the x-ray image system. The regions in the image lying outside of this ROI (region of interest) are nevertheless recognizable, albeit with reduced contrast. This technique in fact effects a significant dose reduction in the border regions of the image, but can be adapted only with difficulty to different subject shapes and sizes. Even with the use of such a filter technique, the dose at specific regions of the body to be examined is locally higher by multiple times more than would be necessary for a good contrast. This problem particularly occurs in body regions in which regions of much stronger x-ray absorption and regions of much weaker x-ray absorption lie next to one another. Since the diagnosing physician must normally examine all organs of an x-ray image, the applied x-ray dose is set such that a sufficient signal-to-noise ratio is achieved for all objects acquired in the image.
Apparatuses for spatial modulation of the radiation field that are positioned between the x-ray source and the patient are known in the field of x-ray imaging in which one-dimensional radiation fields are used in the form of fan-shaped x-ray beams for exposure such as, for example, in conventional computed tomography. In these apparatuses, for example, tongue-shaped attenuation elements are arranged in the form of a one-dimensional array corresponding to the one-dimensional extent of the radiation field. The attenuation elements can be controlled via separate actuators independently of one another, such that individual sections or channels of the one-dimensional radiation field can be weakened or modulated independently of one another by the introduction of the attenuation elements. Such an apparatus is known, for example, from U.S. Pat. No. 5,044,007, in which the attenuation elements are fashioned tongue-shaped and tiltable, and each can be tilted into the radiation field by its actuator. The control of the individual actuators ensues dependent on the x-ray radiation exiting from the body after irradiation of the body to be examined, relative to the respective channel that can be influenced with the attenuation element. The radiation dose necessary for a sufficient contrast can be locally reduced in this manner to the respectively necessary value, such that overall a reduced radiation exposure results for the patient.
Similar apparatuses are known from U.S. Pat. Nos. 5,054,048, and 4,715,056, and European Application 0 251 407. In U.S. Pat. No. 5,054,048, the attenuation elements are designed as sliding elements that are moved into or out of the beam by a sliding mechanism with an electromechanical drive. The attenuation elements are wedge-shaped, such that different degrees of attenuation can be achieved by displacement thereof perpendicular to the beam direction.
European Application 0 251 407 suggests the use of planar attenuation elements made from a piezoelectric material that can be tilted between two positions by the application of an electrical voltage.
From U.S. Pat. No. 4,715,056, a further one-dimensional attenuation apparatus is known in which tiltable pivotable planar attenuation elements are formed from a piezoelectric material as flex transducers that can be bent into the beam path by the application of an electrical voltage. This document furthermore discloses the possibility of an electromechanical drive as well as drive by means of a step motor. In the design with the electromechanical drive, the position of the attenuation elements is derived from the current strength of the current flowing through the electromagnet, namely the activating current.
A further apparatus for spatial modulation of a two-dimensional x-ray field is known from the German Application 102 21 634 (published after the priority date of the present application). In this apparatus, flex transducers (arranged in the form of a grid) to which self-supporting planar attenuation elements are attached, are aligned such that the attenuation elements stand in the beam direction. A minimal beam attenuation ensues in this position. By deflection of individual flex transducers via an electrical control, the attenuation at this location can be specifically increased. Knowledge of the position of each individual attenuation element during the image acquisition is necessary to damp the fluctuation behavior of the flex transducer and for image post-processing. The detection of the current position of each attenuation element ensues in this apparatus with an optical measurement arrangement that detects light passing through the grid of attenuation elements. For this purpose, a light source is necessary at the input side of the grid and a light deflection device is necessary at the output of the matrix. The respective positions of the attenuation elements are determined and evaluated by the shadowing thereof on a photodiode array caused by the attenuation elements. Errors can occur with such an optical detection due to light scattering and image blurring. Furthermore, light channels must be present in the grid mounting in order to be able to conduct the optical projection onto the photodiode array.